1. Technical Field
The present invention relates to wireless communications and, more particularly, wideband wireless communication systems.
2. Related Art
Modern wireless RF transmitters for applications, such as cellular, personal, and satellite communications, employ digital modulation schemes such as frequency shift keying (FSK) and phase shift keying (PSK), and variants thereof, often in combination with code division multiple access (CDMA) communication. Independent of the particular communications scheme employed, the RF transmitter output signal, SRF(t), can be represented mathematically asSRF(t)=r(t)cos(2πfct+θ(t))  (1)where fc denotes the RF carrier frequency, and the signal components r(t) and θ(t) are referred to as the envelope and phase of SRF(t), respectively.
Some of the above mentioned communication schemes have constant envelope, i.e.,r(t)=R, and these are thus referred to as constant-envelope communications schemes. In these communications schemes, θ(t) constitutes all of the information bearing part of the transmitted signal. Other communications schemes have envelopes that vary with time and these are thus referred to as variable-envelope communications schemes. In these communications schemes, both r(t) and θ(t) constitute information bearing parts of the transmitted signal.
The most widespread communication standard in the area of wireless personal area networks (PANs) is currently Bluetooth. This communication standard employs Gaussian minimum shift keying (GMSK), which is a constant-envelope binary frequency shift keying (FSK) modulation scheme allowing raw transmission at a maximum rate of 1 Megabits per second (Mbps). While standard Bluetooth is sufficient for voice services, future high-fidelity audio and data services demand higher data throughput rates. Higher data rates can be achieved in the specification of the Bluetooth Enhanced Data Rates (Bluetooth EDR) standard by selectively applying a variable-envelope 4-level or 8-level phase shift keying (PSK) modulation scheme. With these variable-envelope communication scheme options, the maximum bit rate is increased 4-fold or 8-fold, respectively, compared to standard Bluetooth, while the chosen pulse shaping, a square-root raised cosine filter with a roll-off factor of 0.4, ensures that the RF carrier bandwidth is the same as that of standard Bluetooth, allowing for the reuse of the RF frequency channels.
A transmitter appropriate for a variable-envelope modulation scheme in the Bluetooth EDR standard is a polar transmitter. In a polar transmitter, digital baseband data enters a digital processor that performs the necessary pulse shaping and modulation to some intermediate frequency (IF) carrier fIF to generate digital envelope (amplitude-modulated) and digital phase-modulated signals. The digital amplitude-modulated signal is input to a digital-to-analog converter (DAC), followed by a low pass filter (LPF), along an amplitude path, and the digital phase-modulated signal is input to another DAC, followed by another LPF, along a phase path. The output of the LPF on the amplitude path is an analog amplitude signal, while the output of the LPF on the phase path is an analog phase signal. The analog phase signal is input to a phase-locked loop (PLL) to enable the phase of the RF output signal to track the phase of the analog phase signal. The RF output signal is modulated in a non-linear power amplifier (PA) by the analog amplitude signal. Thus, in polar transmitter architectures, the phase component of the RF signal is amplified through the non-linear PA while the amplitude modulation is performed at the output of the PA.
To produce the appropriate RF output frequency, various frequency synthesis methods can be employed in the polar transmitter PLL. One frequency synthesis method commonly used in polar transmitter PLLs is the fractional-N PLL frequency synthesis method. Typical architectures for such PLL frequency synthesizers include so-called “fractional-N” PLLs. In this type of PLL, single oscillator is caused to produce the specified output frequency required for an outgoing radio frequency transmission by dividing its oscillation frequency by a number, N, and comparing that to an accurate known reference frequency. When in lock, the PLL oscillator will oscillate at a frequency equal to N times the reference frequency. The dividend, however, often is not a whole number, and thus the term “Fractional-N” refers to a non-integer dividend that is used in a PLL to produce the desired output frequency. Such non-integer values may effectively be arrived at by interpolation between multiple integer dividends in such a fashion that the average dividend equals the desired non-integer dividend. Typically, a delta sigma modulator is used to perform the interpolation by appropriately choosing the integer dividends to produce the desired non-integer dividend. The “penalty” associated with this interpolation process is phase noise of the PLL output introduced by the delta sigma modulator.
The popularity of traditional fractional-N PLL frequency synthesizers stems from their ability to synthesize frequencies with, in principle, arbitrary precision. However, a limitation of fractional-N frequency synthesizers is their relatively narrowband nature due to the necessity of attenuating the phase noise introduced by the delta sigma modulator interpolation process. Typically, the bandwidth of the PLL is limited to the 150 kHz-200 kHz range for wireless applications such as the Bluetooth EDR standard. However, for Bluetooth EDR, the required signal bandwidth is much wider, on the order of one MHz. As a result, it is not feasible to design a polar transmitter based upon a fractional-N PLL with the conventional narrow bandwidth. Therefore, what is needed is a polar transmitter architecture capable of providing wideband modulation in the phase path while maintaining high accuracy in the final modulated output signal.